A UN watchdog, the Organization for the Prohibition of Chemical Weapons (OPCW), which is overseeing the removal of Syria's toxic arms, has sent a mission into the war-torn country to investigate.

France and other Western nations are currently examining the evidence with the results expected in a few weeks.

- France 'regrets' Obama U-turn -

Fabius complained that President Barack Obama had failed to carry out threatened strikes against the Syrian regime last year after a sarin gas attack near Damascus killed hundreds of people.

"We regret it because we think it would have changed lots of things ... but what is done is done, and we're not going to rewrite history," Fabius reporters.

But a senior US administration official dismissed the criticism, pointing out that -- without airstrikes -- 92 percent of Assad's stockpile has been destroyed by the OPCW in past months.

"There is no evidence that a strike would have allowed us to remove that amount of chemical weapons," the official said.

Citing witnesses and medical staff, Human Rights Watch said Tuesday it has evidence that "strongly suggests" the Syrian government used chlorine gas on three towns in mid-April.

It documented attacks on the towns of Kafr Zita in central Hama on April 11 and 18, Al-Temana in Idlib on April 13 and 18 and Telmans also in Idlib province on April 21. All are areas under rebel control.

Syrian opposition leader Ahmad Jarba was to meet National Security Advisor Susan Rice as part of an eight-day trip to the United States, seeking to persuade the Obama administration to drop its reluctance to supply the rebels with heavy arms.

He has specifically called for anti-aircraft missiles in order to be able to combat what he has called "the nightmare" of daily barrel bombings.

Jarba is also likely to attend a meeting of the "Friends of Syria" in London on Thursday, in which US Secretary of State John Kerry and Fabius will also participate.

Syrian opposition UN representative Najib Ghadbian said that the rebels share Brahimi's "frustration with the Assad regime for not engaging constructively in the political process."

Forcing Assad to negotiate "will require concerted international pressure that has so far been lacking," he added.